Update Doxygen doxyfile

[ulmus-scott]

This should help to make the docs more useful.

@linuxdude42 I'm not sure how to test the cmake changes, but I followed what you did.

Checklist

• [x] contribution does not duplicate one of our existing pull requests
• [x] contribution is in a branch rebased against master
• [x] code compiles successfully without errors
• [x] code follows the MythTV Coding Standards
• [x] documentation added/updated/removed where necessary
• [x] commits are logically organised and have good commit messages